1. Field of the Invention
The present invention relates to apparatus and methods for measuring the concentration of iodine-containing compounds in an aqueous solution, such as in plasma or urine.
2. Background of the Related Art
Chronic kidney disease (CKD) is a major medical problem in the United States and the rest of the world. According to the National Kidney Foundation in the U.S. alone 20 million Americans have CKD and at least 20 million more are at risk. Over the next 10 years, the number of patients in the U.S. with kidney failure (end stage renal disease, ESRD) is expected to double. By 2010, the cost of ESRD may exceed $28 billion annually, according to the United States Renal Data System, Bethesda, Md.
Some major causes of kidney dysfunction include hypertension, diabetes, lupus erythematosus, chemotherapy and immuno-suppression therapy. In all these instances, it is essential to have an accurate test of kidney function to determine the most appropriate therapeutic intervention, because preventing or slowing the progression of renal disease through early recognition of impaired renal function can reduce the number of patients with end-stage renal disease. At the same time, accurate testing of kidney function can prevent or reduce the need for dialysis and kidney transplantation, both of which are costly procedures.
Glomerular Filtration Rate (GFR) is an accepted measure of how well the kidneys are removing wastes and excess fluid from the blood. A person's current GFR can be determined by administering certain agents into the blood and then measuring their disappearance from the blood and their appearance in urine. Accordingly, GFR is a direct measurement of kidney function and the value of an individual's GFR has been shown to drop before the onset of symptoms of kidney failure. A decrease in GFR correlates with the pathologic severity of kidney disease. Replacement therapy with dialysis or transplantation is presently considered to be necessary when the GFR decreases below 15 mL/min/1.73 m2 
The level of GFR is the product of the single nephron glomerular filtration rate (SNGFR) multiplied by the number of functioning nephrons (N) in both kidneys, as set out in Equation (1).GFR=N×SNGFR  Equation (1)the level of GFR can be decreased either because of reduced nephron number (as in CKD) or because of a reduction in SNGFR (caused by physiologic or pharmacologic alterations in glomerular hemodynamics). There are several factors that affect GFR, including kidney disease, pregnancy, reduced kidney perfusion, marked increase or deficit of extracellular fluid volume, nonsteroidal anti-inflammatory drug use, acute protein load and habitual protein intake, blood glucose control (in diabetic patients), arterial blood pressure and the use of certain classes of antihypertensive agents.
GFR is estimated from the urinary clearance of an ideal filtration marker, defined by Equation (2).Ci=(Ui×V)/Pi  Equation (2)In this equation, Ci is the clearance of the ideal filtration marker (i), Ui is the urinary concentration of (i), V is the urine flow rate, and Pi is the average plasma concentration of (i) during the time interval of urine collection. If substance (i) is freely filtered across the capillary wall and neither secreted nor reabsorbed, then Ci=GFR.
Inulin fulfills the criteria as an ideal filtration marker, and its urinary clearance has long been considered the “gold standard” in measuring GFR. Normal values for inulin clearance in young men and women are approximately 130 and 125 mL/min/1.73 m2, respectively. These values decline with age by approximately 10 mL/min/1.73 m2 per decade after 30 to 40 years of age.
Although inulin is considered to be the ideal filtration marker, its availability is limited and the protocols for measurement of inulin clearance are inconvenient. Clearance of endogenous filtration markers, such as creatinine and urea, has also been used to assess GFR. Serum creatinine determination has become a mainstay in the standard laboratory profile of renal function because of its convenience and low cost. Nevertheless, serum creatinine remains a crude marker of GFR. Creatinine concentrations are insensitive to detection of mild to moderate reductions in GFR. This is due to the nonlinear relation between concentrations of creatinine in blood and GFR. Use of the serum creatinine level as an index of GFR rests on three important assumptions: (1) creatinine is an ideal filtration marker whose clearance approximates GFR, (2) creatinine excretion rate is constant among persons and over time, and (3) measurement of serum creatinine is accurate and reproducible across clinical laboratories. Although serum creatinine concentration can provide a rough index of the level of GFR, none of these assumptions is strictly true, and numerous factors, such as kidney disease, reduced muscle mass, ingestion of cooked meat, and malnutrition, can lead to errors in estimating the level of GFR from the serum creatinine concentration. In addition, several substances such as, glucose, uric acid, ketones, plasma proteins and cephalosporin may lead to falsely high creatinine values when the Jaffe calorimetric method is used.
Alternative clearance methods that use exogenous filtration markers, such as I-125 Iothalamate sodium, Tc-99m DPTA and Cr-51-EDTA, are simpler and have been used in clinical trials. However, they are inconvenient and expensive because of the use of radioactive material and the need for trained personnel to perform the procedure. Additionally, the use of a radioactive marker is also compromised by the short shelf-life of the agent and a desire to avoid radiation exposure. Thus, there is a need to develop methods that can be used in clinical settings without use of a radioactive marker, yet provide precise estimates of GFR.
Iohexol and Iothalamate are iodinated radiographic contrast media developed for use in diagnostic radiology. Because they do not bind to protein and are totally excreted by the kidney through the process of glomerular filtration, their clearance from plasma after a single injection can be used to estimate GFR in humans. Several analytical methods have been developed for determination of Iohexol and Iothalamate concentrations in plasma and urine. The most commonly used procedures involve high performance liquid chromatography (HPLC) and capillary electrophoresis (CE), but both of these separation techniques are suitable for use only in a laboratory setting as both of the techniques require sophisticated instrumentation, regular maintenance and highly skilled personnel to run the tests.
Another simple clearance method is the X-ray fluorescence (XRF) measurement of iodine. XRF is convenient in its simplicity and capacity for rapid turnaround, which are important in the clinical settings. Drawbacks to the XRF method include high detection limits and relatively large sample requirements (4 to 6 mL of whole blood is recommended). Methods such as inductively coupled plasma-atomic emission spectroscopy (ICP-AES) for measurement of iodine can be used to determine Iohexol and Iothalamate concentrations, however, these methods require sophisticated instrumentation, regular maintenance and highly skilled personnel to run the tests.